a) Field of the Invention
The present invention concerns a process for the formation of a deposit by projection of a coating material on a substrate, comprising the steps of melting, by combustion of an oxycombustible mixture, the solid coating material and pulverizing and projecting the molten coating material by means of a flow of a carrier gas containing at least 90% of at least one inert gas.
b) Description of Prior Art
The thermal projection with a flame, combines a whole group of processes in order to modify the surface properties of a substrate by providing, on this surface, a deposit of a coating material, which is generally metallic. Through the combustion of the oxycombustible mixture, the coating material is progressively brought to its melting temperature and the carrier gas pulverizes the molten material into fine particles which are provided with a strong kinetic energy. The particles in liquid or pasty state hit the substrate which is initially prepared for this operation. The carrier gas actually used consists of compressed air and the yields (ratio between the weight of the coating material truly deposited on the substrate and the weight of the coating material effectively used) typically obtained, are of the order of 55 to 57% for the projection of zinc which is the metal most currently used for the production of anti-corrosive deposits, such as on metallic tubes.
The Applicant has noted that the thermodynamic properties of the carrier gas play an important role on the value of the yield. Thus, the vaporization temperature of the coating material may be rapidly reached for particles of small diameter if the carrier gas has a high thermic conductivity. On the other hand, the formation of oxides on the particles during their travel between the melting zone and the substrate to be coated is exothermic and may thus lead to an excessive evaporation of the material to be projected.